My IRQ pin floats when set-up as an external interrupt input (IRQ)?

Question:

My IRQ pin floats when set-up as an external interrupt input (IRQ) ?

Answer:

These interrupts are detected regardless of whether the corresponding pin is set for input or output. When using a pin for external interrupt input clear its DDR bit to 0 and do not use the pin for chip select output, refresh output, SCI input/output.